The present invention relates generally to the field of lithium batteries. Specifically, the present invention relates to lithium-ion batteries that are relatively tolerant to over-discharge conditions.
Lithium-ion batteries include a positive current collector (e.g., aluminum such as an aluminum foil) having an active material provided thereon (e.g., LiCoO2) and a negative current collector (e.g., copper such as a copper foil) having an active material (e.g., a carbonaceous material such as graphite) provided thereon. Together the positive current collector and the active material provided thereon are referred to as a positive electrode, while the negative current collector and the active material provided thereon are referred to as a negative electrode.
FIG. 1 shows a schematic representation of a portion of a lithium-ion battery 10 such as that described above. The battery 10 includes a positive electrode 20 that includes a positive current collector 22 and a positive active material 24, a negative electrode 30 that includes a negative current collector 32 and a negative active material 34, an electrolyte material 40, and a separator (e.g., a polymeric microporous separator, not shown) provided intermediate or between the positive electrode 20 and the negative electrode 30. The electrodes 20, 30 may be provided as relatively flat or planar plates or may be wrapped or wound in a spiral or other configuration (e.g., an oval configuration). The electrode may also be provided in a folded configuration.
During charging and discharging of the battery 10, lithium ions move between the positive electrode 20 and the negative electrode 30. For example, when the battery 10 is discharged, lithium ions flow from the negative electrode 30 to the to the positive electrode 20. In contrast, when the battery 10 is charged, lithium ions flow from the positive electrode 20 to the negative electrode 30.
FIG. 2 is a graph 100 illustrating the theoretical charging and discharging behavior for a conventional lithium-ion battery. Curve 110 represents the electrode potential versus a lithium reference electrode for a positive electrode that includes an aluminum current collector having a LiCoO2 active material provided thereon, while curve 120 represents the electrode potential versus a lithium reference electrode for a negative electrode that includes a copper current collector having a carbonaceous active material provided thereon. The difference between curves 110 and 120 is representative of the overall cell voltage.
As shown in FIG. 2, upon initial charging to full capacity, the potential of the positive electrode, as shown by curve 110, increases from approximately 3.0 volts to a point above the corrosion potential of copper used to form the negative electrode (designated by dashed line 122). The potential of the negative electrode decreases from approximately 3.0 volts to a point below the decomposition potential of the LiCoO2 active material provided on the aluminum current collector (designated by dashed line 112). Upon initial charging, the battery experiences an irreversible loss of capacity due to the formation of a passive layer on the negative current collector, which may be referred to as a solid-electrolyte interface (“SEI”). The irreversible loss of capacity is shown as a ledge or shelf 124 in curve 120.
One difficulty with conventional lithium-ion batteries is that when such a battery is discharged to a point near zero volts, it may exhibit a loss of deliverable capacity and corrosion of the negative electrode current collector (copper) and possibly of the battery case, depending on the material used and the polarity of the case. As shown in FIG. 2, after initial charging of the battery, a subsequent discharge of the battery in which the voltage of the battery approaches zero volts (i.e., zero percent capacity) results in a negative electrode potential that follows a path designated by dashed line 126. As shown in FIG. 2, the negative electrode potential levels off or plateaus at the copper corrosion potential of the negative current collector (approximately 3.5 volts for copper and designated by dashed line 122 in FIG. 2).
The point at which the curves 110 and 120 cross is sometimes referred to as the zero voltage crossing potential, and corresponds to a cell voltage that is equal to zero (i.e., the difference between the two curves equals zero at this point). Because of the degradation of the copper current collector which occurs at the copper corrosion potential, the copper material used for the negative current collector corrodes before the cell reaches a zero voltage condition, resulting in a battery that exhibits a dramatic loss of deliverable capacity.
While FIG. 2 shows the theoretical charging and discharging behavior of a battery that may experience corrosion of the negative current collector when the battery approaches a zero voltage configuration, it should be noted that there may also be cases in which the active material on the positive current collector may degrade in near-zero-voltage conditions. In such cases, the theoretical charging and discharging potential of the positive electrode versus a lithium reference electrode would decrease to the decomposition potential of the positive active material (shown as line 112 in FIG. 2), at which point the positive active material would decompose, resulting in potentially decreased protection against future over-discharge conditions.
Because damage to the lithium-ion battery may occur in the event of a low voltage condition, conventional lithium-ion batteries may include protection circuitry and/or may be utilized in devices that include protection circuitry which substantially reduces the current drain from the battery (e.g., by disconnecting the battery).
The medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions. Depending upon the medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. Clinicians use medical devices alone or in combination with drug therapies and surgery to treat patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life.
It may be desirable to provide a source of battery power for such medical devices, including implantable medical devices. In such cases, it may be advantageous to provide a battery that may be recharged. It may also be advantageous to provide a battery that may be discharged to a near zero voltage condition without substantial risk that the battery may be damaged (e.g., without corroding one of the electrodes or the battery case, decomposing the positive active material, etc.) such that the performance of the battery is degraded in subsequent charging and discharging operations.
It would be advantageous to provide a battery (e.g., a lithium-ion battery) that may be discharged to near zero volts without producing a subsequent decrease in the amount of deliverable capacity or producing a corroded negative electrode or battery case. It would also be advantageous to provide a battery that compensates for the irreversible loss of capacity resulting from initial charging of the battery to allow the battery to be used in near zero voltage conditions without significant degradation to battery performance. It would also be advantageous to provide a medical device (e.g., an implantable medical device) that utilizes a battery that includes any one or more of these or other advantageous features.